I have a relatively simple suggestion for how to implement an emergency broadcast system.

Allow each witness to 'declare' a message via their node, the same way they declare their 'vote' for HBD APR.

Then, each front-end can set parameters determining how many of the Top 20 witnesses must be declaring the same message before they take action to highlight that message via their front-end (or other methods).

They could also use different display methods depending upon how many witnesses are in agreement regarding a given message.